Dendritic cells are specialized antigen presenting cells [APCs] for initiating T cell-dependent immunity in situ. Dendritic cells also can mediate tolerance, centrally in the thymus and perhaps peripherally as well. This subproject will approach the potential of dendritic cells to silence T cells. We will take advantage of a receptor, DEC-205, that is abundant on Dendritic and certain Epithelial Cells. DEC-205 was isolated and cloned, and has the structure of a DECalectin. DEC-205 mediates adsorptive uptake of rabbit anti-DEC-205 via coated pits and multivesicular endosomes, and efficient presentation to Ig-specific, T cells. When a rat mAb to DEC-205 is injected into mice, the mAb targets to dendritic cells but within a week, the T cells become tolerant to an immunogenic challenge of rat Ig in CFA. Here we will target peptides via DEC-205 in vivo to determine the tissue distribution of the injected peptide and its tolerizing activity. We hypothesize that thymic tolerance would ensue following presentation by medullary dendritic cells and possibly cortical epithelium, while peripheral tolerance may develop because of targeting to immature subsets of dendritic cells or by charging these APCs with very high doses of peptide. The dose requirements and duration of tolerance to rat anti-DEC- 205 will be compared with rat mAbs targeted to other cells and dendritic cell constituents. An I-Ea peptide that binds to I-A/b and is recognized by the Y-Ae mAb will be coupled to anti-DEC-205 and used to visualize processing and targeting in vivo, as well as tolerance to I-Ea peptide. Likewise an influenza HA peptide will be coupled to anti-DEC-205 to follow the response of central and peripheral T cells in HA-specific TCR transgenic lines. In parallel, we will screen a large panel of glycoproteins and nonglycosylated proteins for presentation via DEC-205 and carry out structure-function analyses of the DEC-205 domains that are involved in presentation. Preliminary data show presentation of fetuin by DEC-205 transfectants. Glycoprotein tolerance in vivo will be assessed. Finally the effects of the genetic deletion of DEC-205 in marrow and nonmarrow derived APC compartments will be documented. Within the context of this program, we will therefore identify active ligands for DEC-205 and test if targeting MHC II products via this receptor provides a new molecular approach for autoimmune intervention. By targeting autoantigens to critical subsets of APCs, this approach aims to silence disease-producing T cells in type I diabetes and other autoimmune diseases.